The present invention relates to putting sleeves, in particular heat-shrink sleeves, onto articles that are being conveyed, the articles carrying their sleeves subsequently passing through a shrinking oven.
To put heat-shrink sleeves on articles being conveyed, it is conventional to use a technique whereby the sleeves are cut to length from a continuous sheath passing over a shaper for opening the sheath, which shaper is held in a floating position by co-operation between outside wheels and parallel-axis backing wheels carried by the shaper, which outside wheels serve to cause the sheath to advance over the shaper in a generally vertical direction towards cutting means.
For the technological background, reference can be made, for example, to the following documents: EP-A-0 368 663, EP-A-0 366 267, EP-A-0 048 656, U.S. Pat. No. 4,565,592, and JP-A-57 36612.
In some circumstances, the above general approach has been associated with second wheels provided downstream from the cutting means to eject the cut-off segment of sheath onto the article which is brought into register with the shaper by conveyor means for moving the articles stepwise. There are then first outside wheels for causing the sheath to advance over the shaper, and second outside wheels serving to eject the cut-off segment of sheath onto the article in question. All of the outside wheels are naturally motor driven and various types of arrangement have been used to motorize them.
In a first type of arrangement, proposals have been made for the motorization of the second wheels to be completely independent from that of the first wheels so as to allow the second wheels to rotate much faster than the first, thereby causing the cut-off segment of sheath to fall vertically and quickly onto the article in question. That approach is illustrated by document EP-A-0 109 105.
In that document, an article sensor cell is used to respond to an article coming vertically below the vertical shaper by triggering operation of the motors that drive the first wheels and by triggering separate operation of the motors that drive the second wheels so as to cause the sheath to advance over the shaper, so as to cause the sheath to be cut, and so as to cause the cut-off segment to be ejected onto the article. In theory, that approach enables the rate of throughput of the apparatus to be increased, however throughput remains limited because the cut-off segment of sheath is purely and simply ejected onto the article without its downward movement being controlled in any way, and because the articles are delivered, e.g. by means of a worm screw, in independent manner without any synchronization with triggering the descent of the cut-off segment. Furthermore, with certain complicated shapes of article, it is necessary with such apparatus to provide a sleeve of diameter that is much greater than the maximum diameter of the article, thereby considerably restricting control over the quality of sleeve shrinking. When high rates of throughout are reached, it is found that some of the sleeves are incorrectly positioned on the articles, particularly when the sleeves are of considerable height, and even that some articles reach the downstream end of the conveyor apparatus without being provided with a sleeve at all.
In a second approach, still using the same principle of first outside wheels moving the sheath downwards and second outside wheels located downstream from the cutting device and ejecting a cut-off segment of sheath onto an article, rotary drive of the first and second wheels is synchronized. This is well illustrated by document EP-A-0 000 851.
In that document, the ejector wheels disposed downstream from the cutting means are mechanically linked to the first ejector wheels for lowering the sheath, with these wheels being rotated by a single electric motor. Under such circumstances, the various movements are necessarily properly synchronized, but there are limits on possible rates of throughput that are inherent to the fact that the same motor serves to lower the sheath, to cut the sheath, and to drop the cut-off segment of sheath. If the travel speed of the articles is increased, and particularly if it is desired to use sleeves of diameter that is hardly any greater than the maximum diameter of the article, it becomes difficult to conserve the accuracy with which the segments of sheath move down onto the articles, and indeed the articles can also suffer from problems of stability at high rates of throughput. Another drawback of that apparatus lies in the imposed constant speed which is inherent to using a single motor, and which can never vary if the rate of throughput of the machine is modified. It is therefore not possible either to vary the normal operating speed, or to take account of the upstream charge of conveyed articles. In addition, it should be observed that the profile of speed variation for an electric motor as used in document EP-A-0 000 851, and also as used in the above-described document EP-A-0 109 105, is always in the form of a squarewave, with a steep rising slope when the wheels are started, a horizontal plateau for the constant operating speed, and a steep falling slope for stopping the wheels and bringing them to rest. Such a squarewave type of profile with two sudden changes when getting up to constant speed and when stopping inevitably gives rise to shaking which disturbs the proper operation of the process of putting the sleeves into place once high rates of throughput have been reached. Such shaking makes it impossible to use sleeves having one or more lines of microperforations making them easier to open. This problem becomes even worse when it is desired to obtain cut-off segments of considerable height, since more power is then required of the drive motor, thereby further increasing the amount of shaking.
An object of the invention is to improve a machine of the type described in document EP-A-0 000 851, by conserving the principle of synchronizing sheath advance over the shaper and ejection of the cut-off segment of sheath onto the article in question, using control means and motorization means in such a manner as to avoid encountering the above-described drawbacks both concerning the accuracy with which cut-off segments of sheath are transferred onto the articles, and concerning the constraints that arise at the speed used, thereby naturally opening the way to extremely high rates of throughput, even when using sleeves that are very long.
According to the invention, this problem is resolved by apparatus for placing sleeves onto conveyed articles, said sleeves being cut from a continuous sheath passing over a sheath opening shaper which is held floating by co-operation between first outside wheels and parallel-axis backing wheels carried by said shaper, said first wheels causing the sheath to advance along the shaper up to cutting means, second outside wheels being provided downstream from the cutting means to eject the cut-off sheath segment onto the article fed into register with the shaper by stepper conveyor means for the articles, said first and second wheels being rotated by associated electric motors controlled synchronously by a common electronic programmer arranged to determine a continuous profile for speed variation of said motors so as to control the transfer of each sheath segment onto the corresponding article, and said programmer including at least one electronic control card which co-operates with an adjacent encoder mounted at the end of a shaft rotated by a central motor and gearbox unit.
Thus, organizing rotary control of the first and second wheels on the basis of a common encoder and at least one electronic card for controlling the shafts, makes it possible to achieve accurate synchronization for the first and second wheels, with very accurate control over the transfer of each segment of sheath onto the corresponding article.
Preferably, the continuous profiles for speed variation of the electric drive motors of the first and second wheels are bell-shaped. This bell shape, which can come close to being a sinusoidal profile, is very far removed from the squarewave profile encountered in the above-described prior art machines. The absence of sharp corners in bell-shaped profiles eliminates shaking, even when rates of throughput become very high. In addition, this remains true if it is desired to vary speed during normal operation, providing bell-shaped profiles continue to be used.
It is then preferable for the profile associated with the second wheels to be taller than the profile associated with the first wheels in the central zone of said continuous speed-variation profiles. This makes it possible to have a first sheath segment transfer stage at a speed that increases slowly so as to bring the sheath segment accurately onto the insertion portion of the article concerned, and then to make up for lost time by ejecting the sheath segment very quickly, and then ending transfer very slowly so as to stop the sleeve engaged on the article very accurately in the desired final position for said sleeve.
It is also advantageous to provide for the common electronic programmer to have in memory a series of pairs of bell-shaped profiles which are preprogrammed. This makes it possible to have a large number of pairs of preprogrammed bell-shaped profiles as a function of operating conditions, in particular as a function of desired sleeve lengths or as a function of the types of heat-shrink plastics material used, and also a function of the speeds at which articles are conveyed.
In an advantageous embodiment, the apparatus for placing sleeves has a pair of first wheels driven by a common electric motor, and a pair of second wheels each driven by a respective electric motor one of which is slaved to the other.
Provision can also be made for the sleeve-placing apparatus further to include third wheels disposed immediately downstream from the cutting means, said third wheels being mechanically coupled to the first wheels to rotate at a slightly higher speed than the speed of said first wheels. These third wheels are advantageous for organizing assistance in separating the cut-off segment from the remainder of the sheath situated upstream from the cutting means. Provision is then made advantageously for the apparatus to include a third pair of wheels whose axes are offset from the axes of the second pair of wheels by a distance which corresponds substantially to the length of a cut-off sheath segment.
The control associated with the stepper conveyor means for the articles can use conventional designs based on direct mechanical drive from a central motor and gearbox unit, or more advantageously, use can be made of the principle whereby control is obtained from the rotary encoder that is provided in the apparatus of the invention. Under such circumstances, the above-mentioned common programmer will also include an electronic control card associated with the stepper conveyor means for the articles so as to control the conveyor means from the rotary encoder.